The present invention is related generally to a method of determining the amount of soluble gas contained in a body of molten metal and particularly to a method that provides a direct and accurate measurement of such gas content.
Hydrogen is pressure the most important gas found in aluminum and aluminum alloys. It dissolves in the metal in the atomic state and at any free surface tends to escape to set up equilibrium in accordance with the relation: Concentration of hydrogen in solution is equal to a constant times partial pressure, where partial pressure is the equilibrium pressure of the molecular hydrogen at the free surface of molten metal. If the solubility "SO" of hydrogen at a given pressure (e.g., 760 mm of mercury) is known, then a given gas content "G" will give rise to an internal or equilibrium pressure Pi such that: EQU G=SO Pi/760 (1)
An instrument for measuring equilibrium pressure is disclosed in U.S. Pat. No. 2,861,450 to Ransley. To convert the measurement of equilibrium pressure in the Ransley instrument to a measurement of gas content in terms of cubic centimeters per 100 grams of metal, it is only necessary to know the solubility of hydrogen in the alloy of the metal being measured. The solubilities of the important alloys have been determined, and the solubility increases with increases in the temperature of the molten alloy.
The Ransley device employs active collection of hydrogen from the melt by means of a neutral carrier gas. If a carrier gas, such as nitrogen, is passed through the molten metal, hydrogen diffuses out of the melt into tiny bubbles created in the melt by the flow of nitrogen. After a period of time and repeated circulation of the nitrogen through the molten metal the two gases obtain equilibrium.
The hydrogen collected in the carrier gas is determined, in the Ransley patent, by a hot wire catharometer, the catharometer being comprised of two identical cell structures, each containing a fine platinum wire as the sensing element. One of the two cells, i.e., the measuring cell, is serially connected in the flow of the carrier gas while the other cell is open to the atmosphere. The two cells are electrically connected in a simple bridge circuit, with a small electrical potential (voltage) being applied across the bridge and the cells to heat the platinum elements. The hydrogen gas collected by the carrier gas is directed to the measuring cell. The hydrogen diffuses in the cell and cools the wire filament. This cooling increases the electrical resistance of the platinum and this unbalances the bridge circuit; a meter connected across the bridge circuit reads the out-of-balance condition; this out-of-balance condition is the measurement of equilibrium pressure.
Personnel using the instrument of the Ransley patent must now convert the meter reading to a set of values that determines the actual hydrogen content of the molten metal in terms of cc of H/100 grams of pure aluminum. The conversion is made from charts of conversion factors, including temperature, that function as multipliers to adjust the hydrogen value from that of pure aluminum to the appropriate alloy of aluminum. As explained in detail below, the visual reading of a meter by personnel working in foundries, and then converting and correcting by hand subjects such readings to errors. What is therefore needed is a method by which direct, repeatedly accurate readouts of gas content is obtained from an instrument that measures equilibrium pressure and makes the appropriate calculations automatically.